Energetics of ribonuclease A catalysis. 2. Nonenzymatic hydrolysis of cytidine cyclic 2',3'-phosphate

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Abstract
Various kinetic aspects of the nonenzymatic hydrolysis of 2'',3''-cCMP and cyclic 2'',3''-UMP were studied in order to provide a basis for comparison with the RNase A-catalyzed hydrolysis reaction. Sudies of the pH dependence of the nonenzymatic reaction reveal mechanisms that are first order in hydroxide concentration and second order in hydrogen ion concentration, in addition to a water reaction. The rate constant for the water reaction was very small, equal to .apprx. 2.5 .times. 1--6 min-1. General base catalyzed hydrolysis reactions were also studied with imidazole as the catalyst. At pH values in which both the protonated and neutral forms of imidazole are present, a kinetic mechanism was observed that appears to be second order in total imidazole concentration, thus suggesting that bifunctional catalysis occurs. The activation enthalpy for the hydroxide, hydrogen ion, water and imidazole catalyzed reactions was determined.